1. Field of the Invention
The present invention relates to a mold for use in resin encapsulation molding of small size electrical elements such as semiconductors, various electrical parts, etc. with synthetic resin.
2. Background of the Prior Art
Generally, small size electrical parts such as integrated circuit (IC), large scale integrated circuit (LSI), transistor and the like have been packaged with thermoplastic resins such as phenol resin, epoxy resin, etc. to form a packaged product. This packaging work is performed in mass-production scale by employing a transfer molding press in which a mold for resin encapsulation molding comprising a male mold and a female mold is set. The mold for resin encapsulation molding employed in this work comprises a cavity serving as a resin molding section in which small size electrical elements such as semiconductor devices, electrical parts are encapsulated with a synthetic resin material; a pot for receiving the synthetic resin material and melting the received synthetic resin material by heating; a cull section formed on either a female mold or an male mold which is a counter mold of either the male mold or the female mold on which said pot is formed, in such a manner as to face to an open end face of the pot; and a conveying passage for conveying the molten resin material through which said cull section and said cavity are communicated. Operation of the mold of above construction comprises the steps of placing a small size electical element in the cavity of the mold; clamping the male and female molds; placing the synthetic resin material of tablet shape in the pot and melting the resin material by heating; extruding the molten resin material out of the pot by a plunger disposed slidably in the pot in axial direction; conveying the extruded resin material through the conveying passage by way of the cull section; injecting the conveyed resin material in the cavity under pressure; and curing the injected resin material by heating; thus a product of resin molded small size electrical elements being obtained.
FIG. 8 is a partially enlarged longitudinal view of a mold according to the prior art, and in which reference numeral 1 denotes a male mold, and numeral 2 denotes a female mold. A pot 3 is formed on the female mold 2 in this example, and a cull section 4 is formed on the male mold 1 facing to the upper end face of the pot 3. Runners 5, 5' are respectively formed on the male mold 1 and female mold 2 in connection with the cull section 4. At the tip end of the runner 5', a cavity 7 is formed both on the male mold 1 and the female mold 2 so as to communicate with the runner 5' through a gate 6. Disposed in the pot 3 formed on the female mold 2 is a plunger 8 which is reciprocatingly moved up and down to slidably in the axial direction by a drive source not illustrated, whereby immediately after melting a resin tablet 9 placed in the pot 3 by heating, the molten resin material is extruded out of the pot 3 by the plunger 8 so that the molten resin material may be injected under pressure in the cavity 7 through the cull section 4 and runners 5, 5' by way of the gate 6.
A serious problem, however, exists in that when performing the mentioned molding process employing the conventional mold for resin encapsulation molding, the tablet-shaped synthetic resin material cannot be smoothly and evenly molten in the pot. More specifically, the male mold 1 and female mold 2 are heated in the form of thermal conduction from a heater (not illustrated) embedded in male mold plate and female mold plate, and the resin tablet 9 is molten sequentially starting from its upper and lower ends being in contact with the cull section 4 and plunger 8, getting through the steps of placing the resin tablet 9 in the pot, melting the placed resin tablet 9 and injecting the molten resin in the cavity 7. In other words, first the part "a" being in contact with the cull section 4 heated to the highest temperature begins to melt, then a part "c" being in contact with the upper end face of the plunger heated to a temperature a little lower than the cull section 4 begins to melt, and finally a part "b" not being in contact with any part of the mold and plunger comes to melt. In this manner, the synthetic resin material transformed sequentially into a fluid state due to such sequential melting by heating is extruded out of the pot 3, passing through the cull section 4, and flows in the runners 5, 5', through which finally flows in the cavity 7.
Because of the mentioned resin melting process taking place stepwise, the molten state of the synthetic resin material flowing in the cavity from the cull section through the runners and gate becomes unavoidably uneven, and therefore the molten synthetic resin flowing through the runners may come to be injected in the cavity while the molten state thereof being still partially insufficient, i.e., fluidity thereof being partially poor. In consequence, there may arise such problems as occurrence of bubbles, pinholes and/or voids in the injected synthetic resin material, or wire sweep eventually resulting in poor package quality of the obtained products.